In microelectronic (e.g., semiconductor) device fabrication processes, it is highly desirable to use water having a high purity level, e.g., ultrapure water, which typically contains little or no microorganisms. It is typically important to use highly purified water since long contact times are often required by the wafers being treated with the water during fabrication. Unless the water is highly purified, it may serve as a source of wafer contamination since the water often contains impurities such as water-soluble minerals and particulate matter. Additionally, it may be desirable to introduce the ultrapure water to fabrication processing sites via a line which is isolated from potential sources of contamination.
Conventional methods of obtaining ultrapure water have typically focused on techniques relating to sieve filtering, reverse osmosis, active carbon layer filtering, degassing, and ion-adsorption via ion exchange for removing water-soluble minerals and particulates. Moreover, ultraviolet radiation has been used in an attempt to remove microorganisms such as bacteria from the water.
Difficulties in obtaining ultrapurified water have been experienced with respect to the removal of microorganisms. In particular, it is often difficult to completely remove the microorganisms from the water. Moreover, since microorganisms are usually present in the atmosphere, treated water can be recontaminated. Contamination attributable to microorganisms may be exacerbated by the formation of a biofilm which results in biofouling. As known in the art, biofilm is typically defined as a gel-like substance formed by the interaction of microorganisms and extracellular polymeric materials (EPS). These materials typically grow on a substratum solid-liquid interface in an aqueous environment. A biofilm composition may comprise 70 to 95 percent by weight of water. Typically, between 70 to 95 weight percent of dry materials which are present in the biofilm are organic materials. Microorganisms are usually present inside of the biofilm. The chemical structure of a biofilm can vary according to various factors such as, for example, the specific types of microorganisms present in the biofilm and the environmental conditions of the biofilm. A common chemical structure present in the biofilm is polysaccharide, and a biofilm containing this material is known as glycocalyx.
A biofilm may be spread uniformly over the entire surface of a piece of processing equipment, or it may spread in an intermittent fashion. The biofilm is usually thin with a maximum thickness being in the hundreds of microns. Since biofilm can prevent the diffusion of dissolved oxygen, anaerobic microorganisms may be contained in a biofilm as thin as about 50 to 150 .mu.m.
A biofilm is usually heterogeneous in structure due to the various kinds of microorganisms present therein and due to the ever-changing types of microorganisms present in the biofilm. Nonetheless, the biofilm often exhibits functional homogeneity since the microorganisms typically exist in the form of microconsortia which usually function in a similar fashion.
A biofilm may be able to adapt to a variety of ecological environments. Specifically, a biofilm may contain a number of microorganisms which perform various functions such as, for example, storing nutrition in an aqueous-based system, exhibiting resilience to variations in pH, sterilization, dehydration and the like. The biofilm is able to act as a pool which allows for genetic exchange between microorganisms existing in the biofilm. The microorganisms are therefore able to exist in symbiosis. In view of the above, the biofilm is capable of behaving like a microorganism group which may exhibit an ecological niche relating to, for example, the decomposition of various materials.
With respect to microelectronic device fabrication, the residence time of water passing through a flow line is usually shorter than the proliferation time for a microorganism, which is approximately 2 hours in a nutrient-empty environment. Thus, the proliferation of the microorganism inside ultrapure or deionized water may not impact microelectronic device production quality control. Therefore, quality control may be most greatly influenced by the ultrapure water delivery system, the line of production, or the biofilm grown on the surface of raw materials employed in a microelectronic device fabrication process.
It has been observed that a biofilm is capable of existing in ultrapure water having a resistivity of 180 M.OMEGA..multidot.cm. This is potentially significant since ultrapure water having the above resistivity is often used in fabricating highly-integrated microelectronic devices. It has been found that between 10.sup.7 and 10.sup.11 microorganism cells per milliliter may be present in a biofilm even when the concentration of microorganisms inside the water ranges from 1 to 10 colony forming units (cfu) per milliliter.
Microorganisms which may exist in a biofilm are capable of slowly migrating from the biofilm into the surrounding ultrapure water. These materials may thus serve as a potential contamination source. Therefore, it may be desirable to remove both the biofilm existing in an ultrapure water delivery system and ultrapure water line as well as microorganisms which may be present in the water.
Attempts at removing the microorganisms have generally involved employing biocide to physiologically deactivate the microorganisms. Nonetheless, the deactivated microorganism may still be able to attach to a material surface and serve as a surface for a newly-introduced microorganism to adhere. Moreover, the presence of a single microorganism floating on a biofilm may be troublesome since it is typically difficult to sterilize the single microorganism because it often forms a matrix structure with other microorganisms or the EPS produced thereby.
The amount of formed biofilm and microorganisms associated with the biofilm is often influenced by elements such as types and amounts of available nutrients, water shear forces, and the like, irrespective of the amount of microorganisms present in the water itself. Therefore, it may be of greater importance to remove the microconsortia-containing biofilm relative to sterilization of the microorganism itself. Removal of the biofilm, however, has often been difficult. A conventional removal technique has focused on employing a two-step process. The first step typically involves reducing the attraction between the substance surface or biofilm matrix by applying oxidizing agents, biodipersants, surfactants, enzymes, or a combination thereof. It is intended in the first step that the chemicals not affect the microorganism to eventually be treated. The second step typically involves removing the microorganism deposit, including the biofilm, from the substance surface using a physical technique involving the application of a shear force, ultrasonic energy, and the like.
A chemical sterilizer which may be used in removing a microorganism-forming biofilm should satisfy the following conditions. First, the sterilization should remove the microorganism as completely as possible. Incomplete micro-organism removal may result in the re-proliferation of microorganisms. Second, the sterilizer itself should be removed subsequent to use. In particular, in the event that the sterilizer removal is incomplete, the residual may serve as a contaminant. It is also desirable to remove the sterilizer from an economic standpoint. Typically, the sterilizer is removed by cleaning the equipment or pipeline by using sterilizer-treated ultrapure water. Preferably, an on-line measurement of the density of the sterilizer should be made in conjunction with the above. Third, the sterilizer should not damage the fabrication system components either physically or chemically. Fourth, it is desirable that the sterilizer be relatively safe and easy to handle.
Hydrogen peroxide has been used as a sterilizer in an attempt to satisfy the above criteria. Conventional methods of sterilizing an ultrapure water delivery system using hydrogen peroxide have been employed in Samsung Semiconductor Manufacturing Facilities and Mitsubishi Semiconductor of America Facilities. The use of hydrogen peroxide may be advantageous in that little residue may remain on the equipment after sterilization. Moreover, pipeline corrosion may not be experienced since the hydrogen peroxide decomposes to water and oxygen after the sterilization is carried out.
It has been observed that the sterilization may be effective when the hydrogen peroxide is present in high concentrations at high temperatures. Nonetheless, if the temperatures are excessive, the pipeline used to transport the hydrogen peroxide may become damaged. Thus, there is an increased possibility that an organic or inorganic substance may be discharged from the pipeline. In view of such, it may be desirable to carry out the sterilization at a temperature of about 25.degree. C. In addition, it may be desirable to employ a lower concentration of hydrogen peroxide since a high concentration level may result in increased solution expense and increased sterilizer removal time. Additionally, at higher hydrogen peroxide concentration levels, the reaction of a microorganism and the hydrogen peroxide may result in undesirable gas generation. In light of the above, hydrogen peroxide having a concentration of 1 weight percent is typically used in a sterilization procedure.
A sterilization technique using hydrogen peroxide as described above is often employed in the semiconductor manufacturing facilities. However, with respect to microorganism sterilization in pipelines, complete microorganism removal may not be accomplished by the use of hydrogen peroxide. More specifically, sterilization with hydrogen peroxide may only provide a temporary effect, and thus the microorganism may reproliferate. In addition, using hydrogen peroxide in a pipeline may be difficult in that individual pipelines often tend to have different sterilization requirements.
FIG. 1 shows a conventional ultrapure water delivery system and Table 1 sets forth the results of a corresponding sterilization as measured by an Acridine Orange Direct Count (AODC) technique. In this instance, the AODC technique involved first dying the microorganism colony using acridine orange dyestuff, and then counting the number of colonies using an optical microscope. As shown in Table 1, complete sterilization is not realized using only hydrogen peroxide. At an early stage of the process, sterilization is achieved at a given level. Within three months after sterilization however, the number of microorganisms increases as shown in Table 1.
TABLE 1 __________________________________________________________________________ Number of Microorganisms Present in Various Sections of an Ultrapure Delivery System after Before cleaning Remaining Remaining Remaining cleaning by by Percentage Percentage percentage H.sub.2 O.sub.2 H.sub.2 O.sub.2 remaining After After after (cfu) (cfu) percentage one month two months three months __________________________________________________________________________ filter A 143 14 9.8 29 58 158 inlet B 141 28 19.8 38 83 182 filter C 121 1.81 1.5 45 98 147 outlet D 161 4.68 2.9 32 82 180 wet- E 398 9.84 2.4 89 287 428 bath F 239 60.35 25 127 179 304 G 709 25.03 3.5 74 389 655 __________________________________________________________________________ A, B, C, D, E, F, G, are used for distinction of measuring sites.
Numerous difficulties may exist with respect to conventional sterilization techniques. Since an ultrapure pipeline may require a longer sterilization cycle than does an ultrapure water delivery system, is often required to increase the cleaning frequency for the ultrapure pipeline. Also, it is often difficult to remove biofilm-forming matrices. The sterilization and removal of bacteria from the inside of a polisher (which typically includes an ion-exchange resin) is also typically troublesome and is believed to be attributable to the chemical characteristics of the ion-exchange resin. Furthermore, it is typically difficult to sterilize microorganisms contained within the biofilm since biofilms often tend to form prophylactic layers which are typically difficult to penetrate A single application of hydrogen peroxide as a sterilizer may therefore be ineffective at microorganism removal. Accordingly, repeated applications of hydrogen peroxide are often necessary.
There exists a need in the art for compositions useful for sterilizing water employed in manufacturing microelectronic devices which are potentially able to yield ultrapure water in an efficient manner, along with methods of using the same.